The present application relates to a system for handling short circuits on an electrical network comprising parallel operated units which are droop controlled for active and reactive power sharing and connected to each other via impedances and protection switches for detecting a short circuit on the electrical network and for disconnecting the faulty part of the electrical network.
In electrical networks or grids with a high share of renewable energy sources, classical generators can be replaced by inverters that connect storage devices with the electrical network. In general, inverters can be controlled in a grid feeding or grid forming way. Almost all inverters connecting renewable energy sources with the grid are controlled as grid feeding units. This means that they need a voltage and frequency to synchronize. They cannot run without an existing grid that provides these quantities. The control of grid feeding inverters is designed to inject or consume a certain amount of reactive and active power if active power is available.
In contrast, grid forming inverters provide frequency and voltage. They can build the grid if no AC voltage is available or synchronize to an existing voltage and run in parallel with the grid. If changes in load or generation occur, they share these variations in power in a decentralized way. In a more abstract way, the grid feeding devices can be modelled by a current source that can only feed into an electrical network if a voltage and frequency exists on the network. The grid forming inverters, however, can be modelled by voltage sources providing voltage and frequency.
The grid forming inverters run in parallel in a decentralized manner, synchronized by droop controllers for active and reactive power sharing as known from the document “A. Engler and N. Soultanis, “Droop control in Iv-grids,” in International Conference on Future Power Systems, 2005”. Among other things, this control allows sharing active and reactive power between the units, whereby droop controlled inverters can also run in parallel with conventional units, such as diesel generators or gas turbines as they are almost all controlled in a droop kind of way as well. The synchronization between all the units is still decentralized, i.e. without explicit communication, but only through frequency and voltage. Through this synchronization, the voltage sources need no explicit communication network, allowing a geographical distribution of the units.
FIG. 1 shows a schematic block diagram of an electric distribution network or grid comprising a plurality of droop controlled inverters 2.1-2.N including impedances such as transformers or lines, conventional generators 4.1-4.N including impedances such as transformers or lines, renewable energy sources such as wind power units WP1-WPN and photovoltaic (solar) power units PV1-PVN including impedances such as transformers or lines, and loads L1-LN including impedances such as transformers or lines distributed among the grid the grid in a decentralized way and connected to different bus bars B1-BN which are interconnected by transmission lines TL1-TLN. In the following it is assumed that a short circuit could occur on any of the transmission lines TL1-TLN including their ends, i.e. bus bars B1-BN.
FIG. 2 depicts a schematic block diagram of the part II according to FIG. 1 showing an electrical network or grid comprising conventional generator units 4.1-4.N and a plurality of inverter units 2.1 to 2.N coupled to a common coupling point such as a bus bar B1 and via a three phase electrical transmission TL1 to a load L1 and a renewable energy source such as a wind power unit WP1. The conventional generator units 4.1-4.N include conventional, e.g. synchronous or asynchronous generators 40.1-40.N and are coupled through impedances, denoted by transformers 41.1 to 41.N to the bus B1, but could also be coupled through normal impedances or multiple wound transformers such as triple wound transformers having two windings on the low voltage and one winding on the high voltage side, or connecting two conventional generators with a medium voltage grid through one transformer. The inverter units 2.1 to 2.N comprise DC-storage units 10.1 to 10.N such as e.g. batteries, flywheels, fuel cells or a DC bus, generated by DC/DC converters, grid forming inverters 3.1 to 3.N, running in parallel, transformers 7.1 to 7.N the primary windings of which are connected to the output of the voltage source inverters 3.1 to 3.N and the secondary windings of which are connected to the point of common coupling B1 and via protection switches to the electrical transmission lines TL1 of the electrical network. Furthermore the electrical network comprises one or more protection switches 14, 15 in the electrical transmission line TL1 for detecting a fault and disconnecting the faulty part of the grid.
In the exemplary electrical network, shown in FIG. 2 the grid forming inverters 3.1 to 3.N that connect the storage units 10.1 to 10.N with the common bus bar B1 of the electrical network are coupled through impedances, represented by the transformers 7.1 to 7.N, but could also be coupled through e.g. electrical transmission lines TL1-TLN, impedances or a combination of them.
The load L1 could also be negative in case of a renewable energy WP1 source or a node that contains both, renewable energy generation and load, e.g. WP1 and L1 could be modelled as one load connected to bus bar B2.
In case of a short circuit (SC) on the electrical transmission line TL1, the protection switches 14, 15 detect a fault and disconnect the faulty part of the grid. In most of these cases this detection is done by means of overcurrents that occur during short circuits. Normally, these overcurrents are mostly provided by conventional, e.g. synchronous or asynchronous generators. In order to replace them with grid forming voltage source inverters, it has to be assured that those voltage source inverters are able to provide the short circuit currents that were formerly supplied by the synchronous or asynchronous generators.
Providing these short circuit currents with inverters can be divided into three subtasks, namely into a limitation of short circuit currents, power sharing during short circuit and a provision of short circuit currents for different faults.
The first subtask, “limitation of short circuit currents during short circuit” is necessary to protect the electronic components of the inverters from damage. Hence, their output current has to be limited to a certain value, such that the maximum current of each unit is not exceeded even in case the short circuit occurs directly after the transformer such that the short circuit impedance has the smallest value that will occur. Simply switching the inverter units off is not an option, as the short circuit currents have to be served for a certain period of time in the range of seconds to trip the protection switches of the electrical transmission lines.
The second subtask, “synchronization and power sharing during short circuit”, is necessary because in AC grids the units must stay synchronized in order to achieve power sharing. In normal operation, this could be done by using droop controllers for voltage and frequency as mentioned above. With this power sharing mechanism, the balance between generation and consumption is achieved by inverters that change their power (and share the power variations that occur) in order to maintain a power equilibrium in the electrical network.
The droop concept is intended to ensure load/power sharing in normal parallel operation but it is desirable to maintain this property during short circuit, such that the installation of multiple inverters that run in parallel results in a multiple short circuit current. This is important, because tripping the protection switch requires a certain amount of current that has to be delivered by the inverters. Hence, the (mostly reactive) power must be shared among the inverters, if possible. For power sharing during short circuit, the voltage after the transformer must be non-zero, to allow an implicit communication between the inverters through voltage and frequency. If the voltage after the transformer is zero no communication and, hence no synchronization is possible.
The third subtask, “provision of short circuit currents for different faults”, is necessary because in electrical power networks, different types of short circuits occur. In particular the following five types of faults can be divided into symmetrical and unsymmetrical faults. Symmetrical short circuits occur as a) three phase and b) three phase to ground faults, whereas unsymmetrical short circuits occur as a) two phase, b) two phase to ground and c) single phase to ground faults. For all types of faults, the short circuit current must be provided without an excessive peaking of one or more phases above the allowed voltage or current value before, during and after the short circuit.
There are two types of prior art solutions that can be distinguished by the control mode of the inverter during short circuit. The first type of short circuit handling limits the currents by directly controlling them, i.e. direct control of the output or filter currents of the inverter. This control mode is normally used for grid feeding inverters, that only run if a grid with given voltage and frequency exists.
The second type limits the current by reducing the voltage as function of the currents, i.e. indirectly controlling the currents by reducing the voltage as function of the current. This control mode is normally used for grid forming inverters that can provide voltage and frequency to the grid.
Even though, the two short circuit control solutions are often closely connected to the normal operation mode (grid forming or following), the control modes in normal operation and short circuit can differ whereby a grid forming inverter could, for example, change its control mode in a short circuit case from grid forming to grid feeding mode.
For parallel running grid forming inverters the following three solutions have been proposed to cope with short circuits.
From the document “M. Hauck: Bildung eines dreiphasigen Inselnetzes durch unabhängige Wechselrichter im Parallelbetrieb (PhD thesis, Universität Fridericiana Karlsruhe, 2002)” a nonlinear, piecewise affine approach that reduces the voltage of each phase as function of the current of the corresponding phase is known. As the voltage is only reduced if the current exceeds a certain value, the resulting curves have a high total harmonic distortion for all kinds of short circuits. Also, due to the nonlinear limitation for each phase, the instantaneous power would have a pulsating waveform, even for symmetric short circuits, making it harder to synchronize for the grid forming units during a short circuit.
The prior art document “K. De Brabandere, B. Bolsens, J. Van den Keybus, A. Woyte, J. Driesen, and R. Belmans: A voltage and frequency droop control method for parallel inverters (IEEE Transactions on Power Electronics, vol. 22, no. 4, pp. 1107-1115, 2007)” discloses a virtual complex impedance to limit short circuit currents. In comparison to the solution mentioned before this does not lead to a high total harmonic distortion but carries the need for a voltage controller that compensates the voltage drop during normal operation and so extends the complexity of the system. To prohibit the integrator from increasing the voltage during short circuit it also has to be limited. This results in an unnecessary intricacy that may lead to problems in the design of the controller and the virtual impedances.
The prior art document “F. Salha, F. Colas, and X. Guillaud: Virtual resistance principle for the overcurrent protection of pwm voltage source inverter (Innovative Smart Grid Technologies (ISGT) Conference Europe, 2010 IEEE PES, IEEE, 2010)” discloses also a virtual complex current dependent impedance to limit the SC currents. In comparison to the prior approach, the impedance is current dependent in a sense that its voltage drop depends on the difference between a nominal and the measured current.
All known solutions have in common that they use a virtual impedance to guarantee parallel operation during a short circuit. With an inverter running as a voltage controlled current source this would not be possible.
Existing solutions for grid feeding inverters all lack the fact that they need a voltage to keep synchronized during a short circuit. Also, inverters controlled as current sources at the moment only provide positive sequence current that may lead to overvoltages and a decreased amount of short circuit power to trip protection switches as would actually be possible by the inverter ratings. Synchronization and reactive power sharing can only be guaranteed if voltage and frequency on the terminals of the inverters exists.
The above mentioned solutions for grid forming inverters, controlled as voltage sources offer the following drawbacks of: a high total harmonic distortion; a DC voltage offset for certain operating conditions; non-constant power due to the nonlinearity of the limitation, for all types of short circuits, which has a negative effect on the synchronization and power sharing of the units, and results in the need of an additional controller to compensate the voltage drop over the virtual impedance; and an increased complexity which makes it hard to design the system properly.